The present invention relates to speed limiters for internal combustion engines, and more particularly to speed limiters for small internal combustion engines of the type used to power outboard motors, lawnmowers, snow blowers, generators and the like.
Conventional internal combustion engines comprise a primary and a secondary winding that are inductively coupled with one another. A spark plug is connected across the terminals of the secondary winding, and a switch is used for closing a circuit to enable current to flow in the primary winding and for opening the circuit at a time when the spark plug is to be fired. In battery ignition systems, the closing of the circuit that includes the switch allows battery current to flow in the primary winding. In a magneto-ignition system, an electromagnetic field is induced in the primary winding by an orbitally moving magnet in cooperation with a fixed ferromagnetic core around which the primary and secondary windings are wound. The closing of the switch short circuits the primary winding, thus allowing current to flow in it.
The conventional speed limiter timing circuit typically utilizes a digital timer, such as a 555 timer, that outputs a control signal based on an input signal. Because the timer is digital, the input signal is typically a modified signal that has been conditioned to work with the digital circuitry, but is based on a raw data signal. Thus, because a digital timer may handle a raw data signal directly, it may be necessary to condition or enhance the incoming signal. Such digital circuitry is relatively high in cost and requires more room on a circuit board. The use of a larger circuit board affects packaging of the unit as well, requiring a larger housing that is more bulky.
In the past, conventional control switch typically included a pair of hard metal breaker points that were actuated by a mechanism having a cam that rotated in timed relation to the engine's cycle. Other conventional control means include the use of semiconductor devices such as transistors to control primary winding current in a timed relation with the engine's cycle.
Electronic speed limiters are also known which are an integral part of the engine ignition system. A conventional control switch means includes the use of a silicone controlled rectifier (SCR) of the drive circuit of the ignition system. Such speed limiters have the disadvantage that they cannot be easily retrofitted onto an existing engine's ignition system without replacing at least a portion of the ignition system. Also, such speed limiters cannot be easily added as an option onto a standard production engine ignition system.